The present invention generally relates to system for communicating both voice and data over modems, and more particularly to high speed modems offering robust communication between a central office and a customer premises.
High speed digital modems, such as Rate Adaptive Digital Subscriber Loop (xe2x80x9cRADSLxe2x80x9d) modems, are able to transfer data at high rates over the local loop, because they use frequencies which are significantly higher than the voice band frequencies used in Plain Old Telephone Service (xe2x80x9cPOTSxe2x80x9d). By way of example, speech on a POTS system generally occurs in the frequency spectrum between about 0 Hz (xe2x80x9cDCxe2x80x9d) and about 4 kHz, whereas RADSL modems use the frequency spectrum of between about 20 kHz to about 1 MHz. High speed digital modems generally include error detection circuitry which measures the errors which occur during communications. By making such measurements, they are then able to update their statistical knowledge of the wire pair which extends between the subscriber""s location and the central office. Using that statistical knowledge, the modems can select optimal operating speeds. These modems were originally proposed when it was thought that services, such as video-on-demand, would be desirable.
As modem technology has developed, another need has arisen, in that the Internet has become a popular medium for both personal and work related use.
While the high speeds of RADSL modems seem to be quite desirable, their use of high frequencies mean that they also need to be protected from high frequency noise, such as cross-talk from adjacent channels or adjacent loops in the loop cable binder, as such noise causes them to downwardly adjust their operating speeds. In order to avoid certain types of noise, RADSL modems typically require the use of filters, called POTS filters, together with splitters for isolating Public Switched Telephone Network (xe2x80x9cPSTNxe2x80x9d) equipment from the RADSL modems. Indeed, without POTS filters and POTS splitters, POTS signals directly interfere with the RADSL spectrum below about 20 kilohertz and the RADSL spectrum directly interferes with the POTS. POTS filters and POTS splitters reduce POTS signaling transients from interfering with RADSL data transmission. In addition, the use of the high RADSL bandwidth demands relatively high transmit power, which can cause distortions and dynamic range overload to POTS equipment.
Unfortunately, the manufacture and installation of POTS filters and splitters are expensive, and their use sometimes requires rewiring of the customer premises to ensure that all PSTN equipment is properly isolated from the RADSL modems and computing equipment. Consequently, it would be desirable to avoid the use of POTS splitters and filters, in order to avoid the expense they impose (e.g., purchase cost and possible rewiring of customer premises).
Accordingly, there appears to be a need for a mass market modem which has data transfer rates greater than the 33.6 Kbps attainable by PSTN modems, yet under the rate that requires the addition of POTS filters, splitters, etc. to address noise and deleterious transmission line effects often encountered in high speed DSL modems.
Yet another problem which is manifest in increased Internet access and data communications is the increasingly limited availability to the customer phone line or local loop for its original purpose, i.e., voice communications. Of course, one solution is for a customer to purchase an additional phone line. This, however, imposes an additional cost on the customer. Moreover, unless the line is dedicated by the customer for a specific purpose (which is poor utilization), the second line may not always be available when needed.
Accordingly, there is a need to provide an improved modem that accommodates data transmissions, while simultaneously allowing traditional voice operation of a telephone attached to the same line at the customer premise. It is particularly desirable to have such a modem that does not require the use of costly POTS filters and splitters.
Certain objects, advantages and novel features of the invention will be set forth in part in the description that follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the advantages and novel features, the present invention is generally directed to a method and apparatus for communicating data across a local loop, in a manner that senses and dynamically adapts to the simultaneous transmission of POTS (e.g., voice or PSTN modem) information across the local loop. In accordance with one aspect of the invention, a method is provided for dynamically communicating data over a local loop using a modem comprising the steps of transmitting data in a full-band transmission state, sensing a band-limiting condition, and adjusting the transmission of data from the full-band transmission state to a band-limited transmission state, in response to the sensing step. The step of sensing a band-limiting condition includes both the detection of the onset of a condition indicating that the method should enter the band-limited transmission state, as well as the detection of the cessation of that condition, indicating that the method should enter the full-band transmission state from the band-limited transmission state.
In accordance with the method of the present invention, data may be transmitted by the modem across the local loop at the same time that POTS (e.g., voice or PSTN modem data) information is communicated across the same local loop. A significant aspect of the present invention is the dynamic allocation of the data transmission bandwidth, whereby the invention senses a condition indicative of whether POTS information is being communicated. If so, then the system shifts and/or narrows the data transmission bandwidth to allow for voice communications without interference from or with the data transmission. However, when no POTS information is being communicated, the invention dynamically allocates the data transmission bandwidth to utilize at least a portion, if not all, of the frequency band otherwise used for communicating voice information.
In accordance with the preferred embodiment, the method senses an off-hook condition of a telephone handset of a telephone electrically connected to the local loop. In use, a local loop extending between a customer premises and a central office branches, at the customer premise, to support multiple connections to the local loop. In this regard, the various branches or connections are typically routed throughout a customer premises to phone jacks, such as RJ-11 jacks. Multiple telephones may be plugged directly into these jacks for voice communication across the local loop. Similarly, a modem constructed in accordance with the present invention may be plugged directly into one of these jacks. The off-hook condition is preferably sensed by detecting either a change in impedance in the telephone line, or alternatively, a drop in line voltage across the telephone line.
In accordance with one embodiment of the invention, the full-band transmission state is defined by a transmission frequency bandwidth having a lower frequency boundary of less than about 15-20 kilohertz (and preferably less than 4 kilohertz). In the band-limited transmission state, the transmission frequency bandwidth has a lower frequency boundary of greater than 4 kilohertz. The significance of these values, for purposes of the invention, is that when no voice information is being communicated across the local loop, the transmission frequency bandwidth invades that frequency band generally dedicated to the transmission of voice information (i.e., the 0-4 kilohertz POTS frequency band). When, however, the invention senses that POTS information is being communicated across the local loop, or that there is a demand for the POTS band (e.g., telephone off-hook, ring, etc.), then the embodiment shifts the lower boundary of the transmission frequency bandwidth above the generally 4 kilohertz upper limit of the voice band. Preferably, the lower boundary will be shifted upwardly to approximately 20 kilohertz, to allow sufficient separation between the voice and data transmission frequency bands so that no interference between the two is realized, either by voice information corrupting data, or data transmission being heard in the voice band as noise.
For purposes of the preferred embodiment of the present invention, the precise value of the upper boundary of the transmission frequency bandwidth is not so significant, as it is the dynamic adjustment of the lower boundary and/or the reduced power in POTS mode, that realizes the inventive step. However, it will be appreciated that the upper boundary will generally be greater than 40 kilohertz in order to define a meaningful transmission frequency bandwidth for data transmission. Indeed, in the preferred embodiment, the upper frequency boundary is approximately 80 kilohertz. It is believed that this frequency is low enough that transmissions may be effectively implemented without the need for POTS filters or POTS splitters, and therefore significantly reducing the cost of implementing the inventive system. Signal-to-noise ratio is high to permit reasonable data throughput without excessive power incident on attached POTS devices. Also, premises wiring and subscriber loop stubs do not cause substantive nulls in the frequency response. It will be further appreciated that shifting of the upper frequency boundary is not relevant to the present invention. That is, the upper boundary may be shifted in conjunction with the shifting of the lower frequency boundary, or alternatively, the upper frequency boundary may remain substantially fixed.
It will be further appreciated that depending upon loading, line conditions, and other factors the spectral shape of the band-limited xDSL transmission may be varied to minimize noise, intermodulation products, or other interference within the POTS frequency band. More particularly, it is generally understood that the power density of xDSL transmissions is generally greater than that of POTS transmissions. Merely shifting the xDSL transmission into the band-limited transmission state with a lower cutoff frequency of approximately 20 kHz may not always provide a wide enough guard band to prevent interference with the POTS band. Line loading, line conditions, and other factors (which differ among local loops) factor into this determination. Intermodulation products are another source of noise that often is present within the POTS band. When such noise is present within the POTS band, the band-limited transmission state may be further configured by reducing the power-density of the xDSL transmission. Another, related solution may be to uniquely shape the spectral curve for xDSL transmissions. This, for example, may be done by tapering the lower frequency portion of the curve (i.e., that portion near the approximately 15-20 kHz frequency).
In accordance with another aspect of the preferred embodiment, a modem is provided for communicating data across a local loop. The modem includes an input/output signal line that is electrically connected with the local loop (e.g., plugged into an RJ-11 phone jack). The modem also includes a processor unit that is adapted for operation in one of two states: a full-band transmission state and a band-limited transmission state. The full-band transmission state is defined by a lower frequency boundary at a value below approximately 15-20 kilohertz and an upper frequency boundary generally greater than 40 kilohertz (as discussed above). The band-limited state is defined by a lower frequency boundary greater than 4 kilohertz and an upper frequency boundary greater than 40 kilohertz (which may or may not be the same as the upper frequency boundary for the full-band transmission state). The modem further includes a sensor or other sensing means for sensing that the local loop is in POTS mode (e.g. transmitting POTS information, or preparing to transmit POTS information), and the data signal power and bandwidth are adoptively altered to provide data without out inteferring with the POTS transmission. Upon sensing the band-limiting condition, such as an off-hook condition, the controller causes the processor unit to upwardly shift the lower frequency boundary of the transmission frequency band and operate in the band-limited, or reduced-power, state. Likewise, upon sensing no band-limiting condition (or a cessation in the band-limiting condition), the controller causes the processor unit to downwardly shift the lower frequency boundary of the transmission frequency band, and operate in the full-band transmission state, to maximize data throughput.
In accordance with yet a further aspect of the present invention, a method is provided for simultaneously communicating both voice and data between a customer premises and a central office across a local loop. In accordance with this aspect of the invention, the method comprises the steps of (1) transmitting data between the customer premises and the central office in a first frequency band, wherein the first frequency band is defined by an upper frequency boundary and a lower frequency boundary; (2) allocating a second frequency band for transmitting voice information between the customer premises and the central office; (3) sensing a band-limiting condition; and (4) dynamically shifting the lower frequency boundary of the first frequency band in response to the sensed band-limiting condition. In accordance with the invention, the lower frequency boundary of the first frequency band shifted to at least partially overlap the second frequency band when no band-limiting condition exists. The lower frequency boundary of the first frequency band is further shifted to avoid overlapping with any portion of the second frequency band when the band-limiting condition exists.
In accordance with yet a further aspect of the invention, a modem is provided for communicating across a communication link capable of single-use transmissions and multiple-use transmissions. The term single-use transmissions is used to generally connote that a single transmission or communication is occurring across the link. For example, a single PSTN voice call, or a single data communication transmission. The term multiple-use transmissions is used to generally imply that multiple transmissions or communications are occurring simultaneously. For example, the simultaneous transmission of a data communication and a PSTN voice call. The modem constructed in accordance with this aspect of the invention includes an input/output signal line in communication with the communication link. It further includes a processor unit adapted for operation in one of at least two states, a full-band transmission state and a band-limited state, wherein the full-band transmission state occurs when single-use transmissions are occurring across the transmission link, and the band-limited transmission state occurs when multiple-use transmissions are occurring across the communication link.
It will be appreciated that, in accordance with a broad inventive aspect, the present invention operates by adjusting transmit power between a band-limited transmission state and a full-band transmission state. Generally (but not necessarily always), the full-band transmission state occurs when the communication link is operating in a single-use transmission mode, while the band-limited transmission state generally occurs when the communication link is operating in a multiple-use transmission mode. In accordance with this broad concept of the invention, substantial transmission energy is transmitted by the modem in or near the POTS frequency band, when the modem is transmitting in the full-band state. Conversely, very little (ideally zero) energy is transmitted by the modem in or near the POTS frequency band, when the modem is transmitting in the band-limited state. This allows for simultaneous POTS transmissions (e.g., voice, PSTN modem, etc) in the POTS frequency band, and band-limited modem transmissions.